pgsql: Support multiple concurrent pg_basebackup backups.

Support multiple concurrent pg_basebackup backups.

With this patch, pg_basebackup doesn't write a backup_label file in the
data directory, so it doesn't interfere with a pg_start/stop_backup() based
backup anymore. backup_label is still included in the backup, but it is
injected directly into the tar stream.

Heikki Linnakangas, reviewed by Fujii Masao and Magnus Hagander.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=997b48ed96c476650f87ca75d65c9fc1f6273f5a

Modified Files
--------------
doc/src/sgml/backup.sgml             |    7 +-
doc/src/sgml/ref/pg_basebackup.sgml  |    7 +-
src/backend/access/transam/xlog.c    |  296 +++++++++++++++++++++++-----------
src/backend/replication/basebackup.c |   87 +++++++++--
src/include/access/xlog.h            |   11 +-
5 files changed, 295 insertions(+), 113 deletions(-)